The invention relates to a high power laser system including optical means comprising fiber amplifiers and particularly to such system in which the amplified laser beam is free from phase distortions and aberrations.
The invention further relates to methods for producing the amplified laser beam with fiber amplifiers without phase distortions and aberrations.
An individual fiber amplifier or fiber laser has a limitation on its power output due to laser damage and undesired non-linear effects. In order to increase the power and brightness simultaneously and proportionally, several fiber amplifiers/lasers are combined to produce a single more powerful output laser beam.
Optical systems are known for combining spatial coherent beams from separate amplifiers or amplifying channels by phasing their output beams with the use of a phase conjugation effect in which a technique known as SBS (Stimulated Brillouin Scattering) or a photo refractive phase conjugated mirror (PCM) technique are used to increase the overall power of the laser system as well as compensate for phase distortions and aberrations which the laser beams may encounter along the optical paths in each amplifying channel. However, the SBS technique has only been successful in the case of combining coherent beams from bulk laser amplifiers (preferably slabs in order to avoid depolarization) operating in Q-switch pulse format at a low repetition rate and average power. Neither SBS nor photo refractive techniques are suitable to operate with fiber amplifiers because of either power or pulse format incompatibility and/or inability to compensate for large aberrations and depolarization distortions. Moreover, the photo refractive PCM technique has too long a response time making it impractical for compensation of time variable distortions and vibrations.
An object of the invention is to provide a high power laser system including fiber amplifiers in which the disadvantages noted above with the conventional systems are avoided.
In accordance with the invention, a high power laser system is provided based on coherent combining of fiber amplifiers with a loop type PCM which includes a multi-mode fiber amplifier. The loop PCM technique associated with the fiber amplifiers makes it possible and practical to increase the output of fiber lasers to a multi-kilowatt power level and the inclusion of the multi-mode fiber amplifier in the loop PCM automatically provides a match between the fiber amplifiers and the loop PCM while making the entire laser system one constituted entirely by fiber technology.
An advantage of the loop PCM technique is that it provides a unique combination of characteristics to satisfy the demanding requirements imposed by the coherent combining of high power laser amplifiers and particularly fiber amplifiers. The loop PCM can operate with relatively low threshold levels (approximately 10 W in the CW regime) and has the ability to increase the power to a kilowatt level at any pulse format from Q-switch pulses to CW and with large aberrations ( greater than 100xc3x97DL) and also with arbitrary polarization/depolarization, small frequency shift and short coherence length.
The system of the invention is capable of producing moderate to high power laser output having a variety of applications in various electronic systems.
In a first aspect of the invention, an optical system for producing high power laser beams is provided which includes a coupler having an input for receiving an input laser beam, a first multiplexer connected to receive the input laser beam from said coupler to produce a plurality of beams, a plurality of fiber amplifiers to receive respectively said plurality of beams and amplify the same, a second beam multiplexer receiving the thus amplified plurality of beams to combine said plurality of beams into a single amplified beam, and a loop PCM, including a multi-mode fiber amplifier, receiving said amplified beam. The loop PCM provides a loop path for said amplified beam in which said amplified beam is further amplified and phase conjugated with the input amplified beam thereby eliminating any phase and polarization distortions and aberrations occurring in said fiber amplifiers. The loop PCM produces an output beam which is supplied in reverse direction through said second multiplexer, said fiber amplifiers and said first multiplexers in that order and then through said coupler to exit therefrom as a high power output laser beam.
In another aspect of the invention, a method is provided for producing high power laser beams in an optical system which comprises supplying an input oscillated laser beam to a coupler, transforming said input laser beam into a plurality of beams in a first multiplexer, amplifying said plurality of beams in fiber amplifiers, combining the thus amplified plurality of beams into a single beam in a second multiplexer, passing said single beam in a loop PCM in which said single beam is amplified and is output as an output beam which is phase conjugated with the input oscillated beam, and then transmitting said output beam from the loop PCM back to said coupler through said second multiplexer, said fiber amplifiers and said first multiplexer in reverse direction, and finally outputting at said coupler a high power output laser beam.
In another aspect of the invention, the loop PCM technique is combined with a double-pass master oscillator/power amplifier (MOPA) laser system. This has a number of advantages compared to the use of a conventional adaptive optics system insofar that the loop PCM technique phases the output of the amplifiers and compensates for optical distortions of the amplifiers and of all optical elements in the entire laser system automatically. Consequently, there is no need to use beam profile analyzers, phase measurements and the like as in the known adaptive optics system. The PCM technique also eliminates misalignment and depolarization of the beams. Furthermore, the double-pass amplification provides higher gain and efficiency.